Electric machines and generators in general are becoming more compact due to competitive pressures. The smaller these machines get, the greater the heat density they will generate. Nowadays, with the advancement of power electronics, electric machines are being driven by means of variable speed drive units presenting an additional heat source due to the harmonics supplied by the inverter. When such machines are operated at low speed the cooling performance is further negatively affected. All of these factors lead to a temperature rise in the machine which will greatly affect its performance and life expectancy.
In air cooled electric machines and especially in wind turbine generators most heat losses arise from so called copper losses. The exceeding heat has to be removed to avoid so called “hot spots” or insulation wearing.
Therefore, the stator laminate or stator stack is generally air cooled by blowing air through the end windings into the air gap as well as through radial ducts disposed within the stator. Alternatively, direct water cooling of stators by means of pipes or hollow copper strands is known for this purpose.